Well, the Darwin layer does not bring that much for scientist used to customize Linux distributions for high volume data processing.

Java, which was used a lot on my CERN days for data visualization was kicked out, after having a first class status on the platform.

OpenGL was frozen for several years in version 2.1, while Windows and Linux were having OpenGL 3 and 4 drivers available.

OpenCL although designed by Apple has better support in the other platforms.

The development experience seems to be oriented towards development of iPhone applications.

Finally the Apple tax, which doesn't bring much, when compared with hardware volume licenses plus Scientific Linux installations.